1. Field of the Invention
The present invention relates to cryogenic coolers. More specifically, the present invention relates to linear Stirling cycle cryogenic coolers.
2. Description of the Related Art
For certain applications, such as space infrared sensor systems, a cryogenic cooling subsystem is required to achieve improved sensor performance. Numerous types of cryogenic cooling subsystems are known in the art, each having relatively strong and weak attributes relative to the other types. Stirling and pulse-tube linear cryocoolers are typically used to cool various sensors and focal plane arrays in military, commercial, and laboratory applications. Both types of cryocoolers use a linear-oscillating compressor to convert electrical power to thermodynamic PV power. The implementation of the compression/expansion cooling cycle differs between the two and each type has advantages and disadvantages that make one or the other ideal for a given application.
Long life Stirling-class cryocoolers generally contain a minimum of two linear-oscillating motors, one of which drives a compressor while the other drives the Stirling-displacer. In practice, a total of 4 motors are typically included to provide necessary mechanical balancing and symmetry. Each motor generally consists of a magnetic circuit and a driven motor coil that is mounted on a moving, spring-supported bobbin. The magnetic circuits are typically very heavy due to their composition of steel and rare earth magnets. The physical size of the magnetic circuits varies with cryocooler capacity, however they are typically several inches in diameter and length. Hence, the need for separate magnetic circuits for each coil of a Stirling machine necessitates larger system mass and volume relative to pulse-tube type cryocoolers that do not contain a Stirling displacer motor. By comparison, the drive coils are very lightweight and small in all dimensions; the bulk of the mass and volume penalty resulting from the Stirling displacer motor is therefore associated with the magnetic circuit as opposed to the coil.
In any event, the advantage of Stirling-class cryocoolers is that they are generally more efficient than pulse-tube type cryocoolers, particularly at very low temperatures and over widely varying operating conditions. This is principally due to the fact that Stirling cryocoolers contain a moving Stirling displacer piston that can be actively driven to optimize the gas expansion phase angle, a parameter critical to the underlying thermodynamic cycle. For more on Stirling cryocoolers, see U.S. Pat. No. 6,167,707, entitled SINGLE-FLUID STIRLING PULSE TUBE HYBRID. EXPANDER, issued Jan. 2, 2001 to Price et al. the teachings of which are incorporated herein by reference.
Pulse tubes rely on purely passive means to control this phase angle such that no active control is possible. The efficiency and operational flexibility of the Stirling cryocooler comes at the cost of increased system mass and volume, parameters that many applications are extremely sensitive to. Hence, although Stirling-class cryocoolers are generally more efficient and operationally flexible (efficient over a much wider range of operating conditions) than pulse-tube cryocoolers, their increased mass and volume lessen their appeal in many applications.
In the past, tactical Stirling cryocoolers have partially overcome these downfalls through a design that uses compressor pneumatic pressure to drive the Stirling displacer piston; no magnetic structure or coil is required for the displacer piston in this design. However, this scheme has a serious drawback of its own: the lack of a Stirling displacer piston motor precludes any type of active control of the displacer piston. Its movement is determined solely by the thermodynamics of the system.
This is significant because the ability to actively control the stroke length and phase of the Stirling displacer piston (relative to the compressor piston) is essential to the efficient operation of the cryocooler. For example, given a certain heat load, cold-tip temperature and frequency, the displacer piston will need to be operated at a specific stroke length and phase in order for the system to operate at maximum efficiency. If any of these operational parameters change (cold tip temperature, system frequency, etc), it is likely that the optimum displacer stroke length and phase will change as well.
A Stirling cryocooler with a passive displacer piston can therefore be designed for peak efficiency at a single point of operation. In a similar manner to that of a completely passive pulse-tube cryocooler, the tactical cooler's efficiency will decrease significantly if any of its operating parameters are changed. Changes of this type are very common in a large number of cryogenically cooled applications. Hence, passive-displacer Stirling cryocoolers are often ill suited for use.
Other than a complete elimination of the Stirling displacer motor in some tactical cryocooler designs, no known serious attempts have been made to negate the mass and volume penalty associated with Stirling cryocoolers. While sound mechanical and packaging design practices have been used to help minimize the penalty, Stirling-class cryocoolers are generally much heavier and more voluminous than comparable capacity pulse-tube cryocoolers.
Hence, a need remains in the art for a system or method for reducing the mass and volume associated with Stirling cycle cryogenic coolers.